A Pileup of Coronal Mass Ejections Produced the Largest Geomagnetic Storm in Two Decades

TL;DR

This study analyzes the solar and interplanetary origins of the largest geomagnetic storm in two decades, revealing how complex coronal mass ejections and mesoscale variations influence their geo-effectiveness.

Contribution

It provides new insights into the formation of solar superstorms and the impact of mesoscale CME variations on geomagnetic storm strength.

Findings

The 2024 May storm was a 'perfect storm' of circumstances leading to an exceptional geomagnetic event.

Differences in magnetic field and geo-effectiveness were observed between Earth and STEREO A for the same ejecta.

Contrasting cases of complex ejecta show that magnetic field configurations within active regions determine geo-effectiveness.

Abstract

The largest geomagnetic storm in two decades occurred in 2024 May with a minimum $D_{\rm st}$ of $-412$ nT. We examine its solar and interplanetary origins by combining multipoint imaging and in situ observations. The source active region, NOAA AR 13664, exhibited extraordinary activity and produced successive halo eruptions, which were responsible for two complex ejecta observed at the Earth. In situ measurements from STEREO A, which was $12.6^{\circ}$ apart, allow us to compare the ``geo-effectiveness" at the Earth and STEREO A. We obtain key findings concerning the formation of solar superstorms and how mesoscale variations of coronal mass ejections affect geo-effectiveness: (1) the 2024 May storm supports the hypothesis that solar superstorms are ``perfect storms" in nature, i.e., a combination of circumstances resulting in an event of an unusual magnitude; (2) the first complex ejecta, which caused the geomagnetic superstorm, shows considerable differences in the magnetic field and associated ``geo-effectiveness" between the Earth and STEREO A, despite a mesoscale separation; and (3) two contrasting cases of complex ejecta are found in terms of the geo-effectiveness at the Earth, which is largely due to different magnetic field configurations within the same active region.